1. Field of the Invention
This invention relates to surgical instruments, more particularly to improved tips for phacoemulsification needles ultrasonically energized by phacoemulsification surgical devices.
2. Prior Art
Phacoemulsification (PHACO) surgical instruments are used for the erosion and pulverization of malfunctioning or diseased tissue of the eye, in particular the opaque hardened protein of cataract of the eye. Electrical energy is delivered to an acoustic wave generating hand held transducer that conducts energy into the eye via a thin walled (e.g. 0.050 millimeter) tip. The tips available are hollow and generally have a 1.0 millimeter (mm) outside diameter, 0.90 to 0.91 mm internal diameter. These tips are made of titanium metal and have a beveled end. The end faces of the tips were originally set at a 15 degree angle, but are currently available set at 30 to 45 degree angles. In addition, the tips have been made with thinner walls and oval cross sections to allow easier entry into the eye.
Balanced salt solution is delivered by gravity infusion into the eye via an infusion tube and a silicone sleeve that surrounds the tip. A hydraulic pump aspirates the pulverized material which is carried along by the salt solution out of the eye via the hollow center lumen of the titanium tip.
These surgical instruments have consoles that provide an aspirating pump that removes balanced salt solution from the operative site and carries with it the eroded tissue. These consoles also deliver electrical energy to the transducer hand piece that converts electrical to acoustic ultrasonic energy. A piezoelectric crystal generates vibrations in the 28,00 to 50,000 cycles per second range and these vibrations are transmitted to a threaded on titanium hollow metal tip 24 mm in length and 1.0 mm in width. New designs for such titanium tips have only appeared recently.
A non-vibrating plastic sleeve surrounds the tip, and salt solution is delivered by gravity to the anterior chamber of the eye into which the phacoemulsification tip with its encasing plastic sleeve have been inserted. As acoustic energy is delivered to the tip nearby tissue is eroded, and the aspirating pump then removes the tissue fragments along with a portion of the salt solution.
It is desirable to erode the hard cataract material within the thin transparent capsule that surrounds the lens of the eye to prevent injury to other tissues in the area such as corneal endothelium and iris. To accomplish this, a precise delivery of energy must be delivered by the vibrating metal tip. Sharp edges on the tip can inadvertently tear the capsule or cornea, and allow vitreous gel located deeper in the eye to move forward. This often impairs effective healing and prevents satisfactory visual recovery.
The procedure of using ultrasonic acoustic wave field erosion of the nucleus of the lens of the human eye is being utilized more frequently. Typically, a hand held transducer of the type described above is used in these procedures. The hand held transducer converts alternating electrical current into acoustic waves, and is a complex and powerful device. The basic mechanism for this energy conversion is well understood by electrical engineers and physicists.
In spite of this understanding by electrical engineers and physicists, and the large industrial use of ultrasound in chemical and material processing, clinical medicine, and cleaning procedures, there has been almost a complete lack of review materials on the underlying principles from which ultrasonic effect originates. This observation is especially true concerning how ultrasound works within the human eye.
Designed in the 1960's by the Cavitron Corporation in association with Charles Kelman, M.D. of New York City, the erosion mechanism is generally believed to be a mechanical "jack hammer" cutting action by the soda straw-like metal tube having an oblique end, and being ultrasonically vibrated. More specifically, the sharp titanium tip is ultrasonically vibrated and acts as a sort of hollow jack hammer that cuts into and mechanically disrupts the cataract nucleus. This approach has lead to the development of tips having sharp edges and thin walls to better "cut" the cataract. This "jack hammer" concept is the prevailing view of how the phaco device erodes or emulsifies tissue.
In these prior art tips, there exists no structure, or means for focusing the acoustic wave front. Specifically, the thin wall of one of these tips terminates to a small circular end face or rim of approximately 0.050 to 0.1 mm in thickness, the end face being set obliquely to the longitudinal axis of the tip. The geometry of this tip is defined by a flat planar surface of the end face intercepting the cylindrical outside surface of the tip.
This surface geometry does not focus wave energy, but only generates waves normal to the flat planar surface of the end face and diverging waves from the outer cylindrical surface of the tip. Thus, these prior art tips may require to some extent actual contact with the tissue to carry out the "jackhammer" effect. Accordingly, these prior art devices are manufactured with shape edges to more effectively cut tissue. Further, sharpened tips suggest and have resulted in the present thin walled structured tips to increase the penetrating ability of the leading edges of these tips, similar in concept to needle designs for puncturing skin tissue.
Acoustic wave energy physics research done since the 1960's reveals possibilities of other mechanisms for tissue erosion with improved tip designs. Upon careful evaluation of the acoustic energy literature, it is now believed that even the prior art tips do not have to actually touch the cataract nucleus during phacoemulsification to effectively remove tissue. Instead, the energy that erodes the nucleus is created by clouds of millions of acoustic wave generated 80-150 micron sized bubbles by the surfaces of the tip being ultrasonically vibrated. The micron sized bubbles are generated at the end of the metal rim (acoustic horn), and expand and implode within a few acoustic cycles creating massive shock waves (500 atmospheres) plus fluid waves at 400 km/hr.
These micro bubbles have been photographed by B. Svensson of Sweden in a plexiglas chamber, and these photos have been shown at the meeting of the American Society of Cataract and Refractive Surgery held in Boston, Mass. in April 1991. At that meeting, a paper also documented sonoluminescent (flame) activity at the tip of phaco devices. This phenomenon has also been photographed in the past and is well illustrated in the ultrasound acoustic literature. These imploding microbubbles, called "transient cavitation" in the physics literature, generate the energy that erodes any solid surface in the area when an acoustic cloud is released into fluid.
Phaco transducers cause the hollow titanium acoustic focusing horn to move back and forth approximately three (3) microns at a frequency selected by the designer believed to be most efficient for cataract nucleus pulverization. The most efficient types of phaco transducers generate acoustic fields primarily at the phaco tip with little loss laterally. This acoustic energy wave generates within a few cycles (in liquids) the bubbles of gas approximately 150 microns in size. These bubbles release large amounts of energy when they implode at the speed of sound and the process is known as "transient cavitation." These unstable microbubbles implode toward any solid surface in the area. The implosion generates shock waves of approximately 500 atmospheres (1 atm=14.9 lb/sq. in.), and fluid waves of 400 km/hr, plus temperatures of 5,500 Celsius within the bubble, especially if the sonicated fluid contains hydrocarbons.
A second form of cavitation is called "stable", implying some micron sized bubbles that last hundreds or thousands of acoustic cycles. Their activity is less well understood by researchers. The massive energy released by cavitation erodes the transducer tip necessitating that they be made of a metal such as titanium.
Even though surgical procedures involving the use of phacoemulsification surgical instruments having proven effective, there is some risk of phaco thermal injury to the anterior segment of the eye during the procedure. The implosion of microbubbles during the process generate massive fluid and shock waves that erode the solid material cataractous nuclei, and can release excess thermal energy into the eye. Further, residual heat from the phaco transducer is conducted down the hollow titanium needle (acoustic focusing horn) and radiates in the anterior chamber potentially causing thermal damage within the anterior segment. Piezoelectric transducers are more efficient and conduct less heat along the needle compared to older magnetostrictive type transducers.
To prevent heat damage, a constant flow of balanced salt solution in and out of the anterior segment is needed to transfer heat out of the eye and to remove lens debris (lens milk) so that the surgeon can visualize the area. However, any problem with proper balanced salt solution circulation can quickly result in heat damage to eye tissue. To insure proper circulation, it is recommended that the surgeon should personally:
1. Visually be certain that balanced salt solution (bss) is being aspirated from the transparent test chamber into the catchment device, that the test chamber remains filled or only slightly dimpled when the device is in phaco mode and held a eye level, and that bss exits from the silicone infusion ports before the device is placed in the anterior chamber;
2. Kink the infusion line while in phaco mode and watch for the test chamber to collapse. Follow this by kinking the aspiration line and listen for the sound of vacuum build up;
3. Ascertain that the incision is large enough for the phaco transducer tip being used, thus avoiding pinching the silicone infusion sleeve, and that some bss leaks from the incision;
4. Aspirate some viscoelastic, if present, from the anterior chamber before entering phaco mode to guarantee that balanced salt circulation not be impaired;
5. Avoid overtorquing the incision (greater tendency if made in the cornea) such that the silicone sleeve is compressed against the edges of the incision;
6. Be aware prolonged time in phaco mode delivers more heat via the titanium tip (use short bursts of phaco power during carving of the nucleus and consider use of pulse mode if available;
7. Become aware of venting sounds that many machines emit if aspiration is impaired; and
8. Watch for persistence of "lens milk", a whitish material of lens fragments in the area of the phaco tip, suggesting movement of bss is restricted. Rigid titanium infusion sleeves have been promoted to guarantee bss is infusing readily and that bss can leak from the incision. However, if these are malaligned they may be frayed by the phaco tip oscillations releasing metal fragments into the eye. The best prevention of thermal injury is to be aware that all transducers lose some energy as heat that is conducted via the titanium tip and that circulation of bss is essential to prevent thermal injury.
Other means for reducing the risk of heat damage can be provided by designing transducers with thermal sensors that stop the device if overheating occurs. Balanced salt solution is currently being chilled prior to its use during the phaco procedure. It could be circulated through larger channels in the transducer handle to create more cooling. It has been traditional for the acoustic horn titanium tip to have a thin wall with the tip bevelled between 15-45 degrees with a 0.91 mm lumen. This design has been used since the 1960's, and could be redesigned to create more efficient acoustic wave fields at the tip, thus eroding the nucleus with less energy, thereby reducing the risk of thermal and or chemical injury.
Researchers are studying the effects the enormous heat generated within liquids can have in forming new chemicals (sonochemistry). The phenomenon of the flame generated within the bubble is known as sonoluminence. This heat is rapidly dissipated and does not significantly contribute to raising the temperature of the liquid being sonicated. Sonochemists are aware that water is broken down to H.sub.2 O.sub.2 and free OH radicals in ultrasonic acoustic fields generated by transducers with designs similar to those used in ophthalmic surgery. It has been demonstrated that these reactions are occurring in the eye during phacoemulsification (See Svensson, Eur. Soc. Cataract and Retract. Surg., September 1991).